Storage tanks are widely used industrially and by governmental bodies for the storage of many different liquids. Many of the stored liquids are highly flammable, such as gasoline, benzene, and diesel fuel, as well as liquids obtained by liquefaction of otherwise normally gaseous materials, such as liquefied natural gas, propane, butane, ethylene and propylene. Storage tanks for such materials generally are surrounded by a moat or trench defined by a dike spaced away from the tank. Were such a tank to fail and stored liquid escape, the liquid would flow into the moat and be prevented from spreading any further. While such a safety approach is predicated on containing the liquid and any resulting fire, it assumes a willingness to sacrifice the entire liquid contents of the tank. This is undesirable from a safety position because of the extended time needed to burn off the escaped liquid or to recover it if no fire results. It is also undesirable from an economical viewpoint because most of the stored liquids are very valuable now and are increasing in value at least year to year. Accordingly, a need exists for a storage tank of increased containment integrity which will retain nearly all of its stored liquid content if the tank fails and some liquid escapes to the tank exterior.